The vector-borne and often asymptomatic nature of Zika virus infection makes it a more complex public health challenge than Ebola, researchers say.
The Zika virus was first isolated from a macaque in Uganda in 1947, and has historically been restricted to Africa and Asia. However since its introduction to Brazil in 2014 or early 2015, possibly via Polynesia, it has spread rapidly and estimates now point to between 440,000 and 1,300,000 cases of Zika virus infection in Brazil during 2015.
In the past, Zika virus infection in adults has presented with non-life-threatening symptoms including mild fever, maculopapular rash, arthralgia, myalgia, headache, retro-orbital pain and vomiting.
Writing in the March 1 online edition of PLoS Neglected Tropical Diseases, researchers say the viral variant currently associated with the outbreak in Brazil is presenting a new and more challenging public health problem.
“What makes this outbreak a high priority global public health concern is the association with incidence of birth defects involving the central nervous system and the apparent increased incidence of Guillain-Barré syndrome,” wrote Dr. Robert W. Malone, from RW Malone MD, and his coauthors (PLoS Negl Trop Dis. 2016 Mar 1. doi: 10.1371/journal.pntd.0004530).
They cited one retrospective study in French Polynesia that suggested there was a ratio of one case of Zika-associated Guillain-Barré syndrome for every 208 suspected cases of Zika virus infection.
The outbreak has also been linked to an unusually high incidence of the otherwise rare microcephaly, with Brazil recording a 20-fold increase in incidence during 2015. The connection with Zika virus is supported by a case study in which large numbers of viral particles were found in the central nervous system tissue of a microcephalic Zika-infected fetus.
Based on estimates of the overall incidence of Zika virus infection, researchers have calculated that Brazilian mothers infected with the virus are 3,700-11,000 times more likely to deliver infants with primary microcephaly, compared with those who are not infected.
There are still some key uncertainties around the transmission of Zika virus, the authors said.
“The degree to which humans, nonhuman primates, or other animals can amplify and transmit the virus to insect vectors is poorly understood,” they wrote. “The typical range and types of insect vectors observed in the past may not be predictive for the virus now circulating in the Americas [and] infectivity of the circulating strain, viremia levels, duration, and risk of occult persistence are not yet understood.”
The virus is transmitted primarily by mosquito vectors such as Aedes aegypti and Aedes albopictus, with primates – including humans – the best documented animal reservoir.
“Recent reports indicate the potential for both human blood-borne and sexual transmission of Zika virus, including prolonged presence of virus in semen,” the authors wrote.
The virus has also been found in the saliva of infected individuals, and viral sequences have been identified in breast milk.Commenting on possible medical countermeasures to combat the spread and impact of the Zika virus, the paper’s authors noted that due to the absence of an existing vaccine and long potential development times for candidate vaccines, other prophylactics and therapeutics need to be explored.
In particular, they called for development and deployment of Zika diagnostics to regional clinical health laboratories, discussions between obstetricians and patients about the risks to ongoing or planned pregnancies, and resources for neurologists dealing with “unprecedented” Guillain-Barré syndrome outbreaks.
“Perhaps the biggest challenge with Zika will be to recognize it for what it is: a new disease which does not fit the epidemiology or response paradigm of Ebola or dengue and which will demand effort, resources, unparalleled collaboration, and above all, open mindedness in formulating responses.”
Two authors declared employment with and equity holdings in RW Malone MD, and two authors declared employment with – and one of these also declared equity holdings in – Nanotherapeutics.